Comparison of structure of gum exudate polysaccharides from the trunk and fruit of the peach tree ( Prunus persica)

Abstract

The peach tree ( Prunus persica) produces gum exudates on its trunk and fruit. The former was extracted with water to give polysaccharide (PPN) and residual material was extracted with alkali giving structurally similar PPNA (NMR examination), which was formed in a slightly higher yield (42% compared with 37%). PPNA consisted of Ara, Xyl, Man, Gal, and uronic acids in a 36:7:2:42:13 molar ratio and was homogeneous on HSPEC with M w 5.61(±0.22) × 10 6 g mol −1. Methylation analysis showed mainly nonreducing end- (20%), and 3- O- (6%) and 4- O-substituted Ara p and/or 5- O-substituted Ara f units (14%) and nonreducing end-units of Xyl p (13%). The core Gal p units were mainly 3,6-di- O- (19%) and 3,4,6-tri- O-substituted (14%). The 13C NMR spectrum of PPNA confirmed its complexity with 5 C-1 signals from δ 107.5–109.5 from α- l-Ara f and a main one at δ 103.2 from the β- d-Gal p units. A controlled Smith degradation of PPNA gave 19% polymer (PPNAS), which was almost completely degraded, in a very low yield, to an inner core of PPNAS2 after a second cycle. This contained mainly β- d-Gal p units, that were nonreducing end- (25%), 3- O- (34%), and 2,3-di- O-substituted units (21%), with no evidence of 6- O-substitution. Partial hydrolysis of PPNAS with 0.1 M TFA at 100 °C removed Ara f units, to give polymeric PPNAS60 (20% yield), which had Gal as its major component (85%). Methylation analysis showed a branched structure mainly nonreducing end- (18%), 3- O- (15%), 6- O- (45%), and 3,6-di- O-substituted Gal p units (9%): its 13C NMR spectrum had a C-1 main signal at δ 103.8 from β- d-linked units. Under stronger hydrolysis conditions β- d-Glc pA-(1 → 6)-αβ- d-Gal and β- d-Glc pA-(1 → 2)-αβ- d-Man were formed. The fruit gum polysaccharide (PPNF) was homogeneous with M w 6.43(±0.64) × 10 6 g mol −1 and according to 13C NMR and HMQC, and TOCSY H-1 correlations, showing few structural differences. These were not apparent on monosaccharide composition and methylation analyses, which only revealed quantitative variations.